With development of communications and a spread of multimedia technology, various large-capacity transmission technologies have been applied to a wireless communication system. As a method for increasing wireless capacity, there is a method of assigning more frequency resources, but there is a limit to assign more frequency resources among restricted frequency resources to many users. One of methods for more efficiently utilizing the restricted frequency resources, there is a method of making a small cell. If the size of the cell is made small, the number of users one base station has to give service is decreased, so that the base station can assign more frequency resources to users. As the size of the cell is made small, it is possible to offer better large-capacity service to many users.
In an institute of electrical and electronics engineers (IEEE) 802.16 task group that chooses the next-generation air interface standards and a nonprofit worldwide interoperability for microwave access (WiMAX) forum that provides service and network standards for a broadband wireless connection system based on IEEE 802.16, standardization has been being achieved for a wireless connection system supporting a femto-cell to enhance efficiency of the system and improve quality of service (QoS) under an indoor environment. The WiMAX forum defines a femto base station as a low-power inexpensive base station connected to an Internet protocol (IP) network through a fixed wireless link or a local broadband wired link. The femto base station is connected to the IP network spread in a home or an office, and accesses a core network of a mobile communication system through the IP network, thereby providing mobile communication service. That is, the femto base station may be connected to the core network of the mobile communication system through a digital subscriber line (DSL). A user of the mobile communication system may receive the service in the outdoors through the existing macro-cell, but receive the service in the indoors through the femto-cell. The femto-cell improves indoor coverage of the mobile communication system by making up for that the service of the existing macro-cell becomes deteriorated in a building, and provides the service intended for the a specific user, thereby providing voice and data service of high quality. Further, the femto-cell can provide a new service not given in the macro-cell, and the spread of the femto-cell may cause fixed-mobile convergence (FMC) to be accelerated and an industrial-based cost to be reduced.
The service of the femto base station is affected by a line error of the IP network, software rebooting, power down, etc. For example, the femto base station may be temporarily turned off or disconnected from the core network of the mobile communication system, and thus the femto base station may be out of service. The WiMAX document ‘Requirements for WiMAX Femto-cell System v.1.0.0’ states that an air interface has to be instantly disabled in the case of abnormal operation of the femto base station. Also, ‘IEEE 802.16m System Description Document (SDD)’ specifies the following for ensuring the reliability of the femto base station. If the communication between the service providing network and the femto base station is disconnected for a certain period of time, the femto base station 1) disables the air interface immediately, 2) presents a way such as a handover to the base station in order to give service continuity to sub users, and 3) notifies a new user not to enter from another cell.
In other words, when inactivating the air interface, the femto base station has to not only notify being out of service but also give a handover command to sub user equipments (UE). Also, the UEs notified by the femto base station about being out of service have to be instantly handed over to a neighboring cell. In this case, a plurality of user equipments performs the handover at the same time, so that the handover may be delayed due to handover procedure in the plurality of user equipments.
Accordingly, a method is required for more efficiently performing the handover procedure as the femto base station is out of service. Also, there is a need of a method for specifically announcing that the service is out, so as to make a user equipment at an idle mode do not camp to a base station of which the air interface is scheduled to be disabled.